The effect of angle of incidence on exterior façade isolation

A common acoustical task is the design and specification of the acoustical performance of the exterior façade system to protect building occupants from road and railroad noise. This involves calculating the outdoor-to-indoor noise reduction of the façade. The incident sound wave from a roadway will often approximate a plane wave from a specific angle of incidence, but the only data usually available is the diffuse field transmission loss of the assemblies as measured in the laboratory. While it is understood that there can be significant differences between the transmission loss at a given angle of incidence compared to the diffuse field value, a detailed and quantitative understanding of the effect is lacking. Measuring the transmission loss from plane waves at varying angles of incidence is difficult, and a combination of theoretical analysis and field measurements may allow a better understanding of the relationship and hence more accurate calculations of the interior noise level from transportation sources. The literature is reviewed, and data from field measurements is analyzed to begin to evaluate this effect.